Look in your BIOS settings. Most motherboards have the option to hold power button for 2-4 seconds to power down.
But you don't want the hard power off.
I think Windows XP has a power down option too. Check control panel to see if you can find it, I cant remember where it is. (I have Win7 on this PC and cant find it, but that doesn't mean it doesn't exist on Win7.)
Of course if none of the above works for you, you could possible use a relay to switch the power button wires to a keyboard input on boot up so that the next power button press will trigger a shutdown in your frontend.
Of course the frontend would have to be setup for this too.
If you have a printer port, you could switch the button to an input on it, using a simple program that monitors that input, press and hold the button for 4 seconds and the programs initiates a windows shutdown.
(I can probably produce a program for you if you want to go this route.)
Lets think on it, im sure there is a simple way to do this.